As fiber-reinforced composite materials, a glass fiber-reinforced resin has been known which is composed of a resin and glass fibers impregnated therewith. This glass fiber-reinforced resin is generally opaque. A method for obtaining a transparent glass fiber-reinforced resin has been disclosed in Patent Documents 1 and 2 mentioned below, in which the refractive index of glass fibers is allowed to coincide with the refractive index of a matrix resin.
The presence of some bacteria that produce cellulose fibers has been known. In Patent Documents 3 and 4, there have been disclosed molded materials having various shapes, such as a sheet, a yarn, and a three-dimensional body, which are formed from cellulose fibers produced by bacteria (hereinafter referred to as “bacterial cellulose”).
The known glass fiber-reinforced resins disclosed in Patent Documents 1 and 2 may become opaque under some working conditions. Since the refractive index of a material has a temperature dependence, even when being transparent under certain temperature conditions, the glass fiber-reinforced resins disclosed in Patent Documents 1 and 2 are changed to translucent or opaque under conditions different from the above temperature conditions. The refractive index of each material has its own wavelength dependence; hence, even when the refractive index of fibers and that of a matrix resin are allowed to coincide with each other at a specific visible light wavelength, a region in which the refractive index is shifted may probably exist in the entire visible light region, and as a result, the transparency cannot be obtained in the region described above.
Bacterial cellulose disclosed in Patent Documents 3 and 4 is composed of monofilaments having a fiber diameter of 4 nm, and this fiber diameter is considerably smaller than a visible light wavelength, so that refraction of visible light is unlikely to occur. However, when being used to form a composite together with a resin, the bacterial cellulose is disaggregated for the use in Patent Documents 3 and 4. When a product produced by bacteria is disaggregated by applying a mechanical shearing force typically with a grinder, bacterial cellulose fibers are closely attached to each other during a disaggregation process to form bundles having a large fiber diameter which may cause refraction and scattering of visible light, resulting in degradation in transparency of a composite material formed using the disaggregated cellulose as described above.
As described above, there has not yet been provided a fiber-reinforced composite material that always maintains superior transparency regardless of temperature conditions and wavelength bands.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-207234        Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-156279        Patent Document 3: Japanese Unexamined Patent Application Publication No. 62-36467        Patent Document 4: Japanese Unexamined Patent Application Publication No. 8-49188        